In order to impart the expected qualities to the tire, the reinforcement belts are generally formed of at least two plies the cables of which form, relative to the circumferential direction, angles having opposite signs, of absolute values which are close to one another but not necessarily identical. Once in place, these plies effect triangulation with the plies of the carcass reinforcement, the cables of which generally form an angle of 90° with the longitudinal direction.
According to the manufacturing processes commonly used in the tire industry, the crown reinforcement plies are laid after the carcass obtained in a first stage has been shaped, that is to say after a toric form has been imparted to the tire blank of substantially cylindrical form. Said carcass generally comprises a carcass reinforcement ply anchored at its two ends to rings for reinforcing the bead.
The belt plies are laid when the diameter of the blank is close to the definitive diameter of the tire. This diameter is calculated to permit insertion of said final blank in the vulcanisation press.
Another method consists of producing the whole of the crown on a cylindrical form referred to as a crown form, the diameter of which is also close to the definitive diameter, and on which are deposited in succession the crown reinforcement plies and the profiled elements constituting the crown section of the tire and comprising, inter alia, the tread. The finished crown is then transferred by a transfer ring to the carcass which, by conforming thereto, adheres to the inner part of the crown.
On the other hand, and still with the aim of improving the performance of the tires, the advantage of producing crown plies the reinforcement cables of which have angles which are variable over their length has been demonstrated. More particularly, the angle of the reinforcing threads relative to the longitudinal direction is less at the centre than on the edges of the ply. The effect of this S-shaped arrangement is to facilitate the flattening of the tire by improving the comfort and the stability, and to reduce the wear phenomena at the ply edge. This type of tire is described by way of example in patent FR 2 037 520 or alternately in patent EP 312 300.
The mechanisms linked to the triangulation of the plies are widely known to the person skilled in the art, and are the subject of numerous publications, among which mention will be made of publication FR 2 797 213 of the Applicant.
A process for producing this particular form of crown belt is set forth in the aforementioned patent FR 2 037 520. It consists of depositing two crown reinforcement plies one on the other, making them adhere strongly over their entire length and over part of their width, and stretching the whole in the longitudinal direction. One variant of this process consists of including a separating element over part of the width, so as to permit sliding of the plies and to define the zones of the ply edge in which it is desired to modify this angle. Once stabilised, the ply is laid directly on the carcass of the tire which has beforehand been curved into its toric form.
It will be observed that this process necessitates assembling the plies on an independent station, but also that producing the join of the two ends of the assembly thus produced may prove awkward owing to the superposition of plies of different angles.
The process described in application FR 2 797 213 makes it possible to produce a crown having the characteristics set forth above, but it is accompanied by the deradialisation of the carcass reinforcement ply beneath the crown zone.